Electrochemical lithium doping into carbon electrodes consisting of multiwalled carbon nanotubes produced by catalytic decomposition of acetylene has been carried out in button cells. Direct observation of the sample by low- and high-resolution transmission electron microscopy reveals that the lithium species are not Intercalated between the graphene shells of nanotubes and that the electrochemical process is reversible. Li-7 NMR and Raman spectroscopies allow us to study the nature of interactions between lithium atoms and the host material and to propose an insertion mechanism that only implies a storage of lithium at the surface of nanotubes.